Gas-tight separation of the cathode chamber from the anode chamber represents a substantial problem with a fuel cell stack. If an operating material from the anode chamber were able to enter the cathode chamber directly (or vice versa), the consequence would be an oxygen-hydrogen reaction.
The problem of tightness is particularly serious in the case of so-called high temperature fuel cells, because, in addition to temperature loading, thermal stresses also result from the heating and cooling of the fuel cells.
A fuel cell stack of the type described above is disclosed in the specification DE 40 09 138 A1. A plurality of openings are incorporated within the bipolar plates, which are adapted to one another in such a manner that supply channels and/or outlet channels for operating materials are formed as a result.
A fuel cell stack in which at least one electrode is sufficiently thick to be self-supporting is already known. In this context, self-supporting is understood to mean that such an electrode substantially retains its form if the horizontally orientated electrode is held by one corner or one edge and lifted. The portion of the electrode which is not held does not bend as a result of gravity.
A self-supporting electrode of this type may, for example, be 1.5 mm thick. In order to save materials and ensure the functional efficiency of the electrode during operation, the electrode should be as thin as possible. In view of this requirement, an electrode for high temperature fuel cells according to the current state of technology must be at least 0.5 mm thick.
The provision of a self-supporting electrode allows the application of a very thin electrolyte layer to the electrode. A thin electrolyte layer is desirable, because this can then be penetrated rapidly and extensively by the ions of an operating material. The performance of a fuel cell can be improved in direct proportion to the number of ions which can pass through the electrolyte layer.
The problem of tightness in a fuel cell stack is particularly difficult with fuel cells of this type which provide a considerable overall thickness in view of the above-mentioned design.
With reference to fuel cells with self-supporting electrodes, it is already known that channels can be provided from which the operating material passes into the individual electrode chambers of a fuel cell stack. By contrast with the prior art, which is known from the specification DE 40 09 138 A1, these channels are formed from separate structural elements. This is understood to mean that openings for the formation of channels according to specification DE 40 09 138 A1 are not provided in the case of fuel cells with self-supporting electrodes.
The provision of separate structural elements increases the problem of gas tightness because, in this case, an additional component must be sealed, which, under some circumstances, consists of a further material, different from that which has already been used. Additional thermal stresses may result from this.
The object of the invention is to create a fuel cell stack of the type named in the introduction, in which tightness is ensured in an improved manner.
The object of the invention is achieved with a fuel cell stack with the features of claim 1. Advantageous embodiments are described in the dependent claims.